I believe I'm seeing this bug in a non-threaded and non-forked env.
System:
OSX 10.11.1 (15B42)
Python 3.5.0 (from brew install)
I'm using aiohttp to create several dozens of HTTP connections to the same server (an async tornado web server). Nothing special is being done around the event loop creation (standard get_event_loop()). However in my system the event loop is frequently stopped, via ioloop.stop(), and restarted via ioloop.run_forever(). I'm not sure this is related to the issue yet, but it's worth mentioning.
I can't provide simplified test code just yet, but I can reproduce in my env with nearly 100% odds when doing a full system test. Attached is a sample backtrace.